The present invention relates to a novel electrically insulating sintered ceramic body suitable as a substrate of, e.g., circuit boards. More particularly, the present invention relates to a novel ceramic body capable of being prepared by a sintering process at an outstandingly low temperature of 1000.degree. C. or below and still suitable as a substrate of electronic circuit boards having high mechanical strengths as well as excellent resistance against migration of a metal when in contact with metal-made parts.
Needless to say, the demand for electrically insulating plates for circuit boards is rapidly growing in recent years as a substrate material of, for example, semiconductor chips along with the progress in the electronic instruments. It is an outstanding trend that the material of these insulating substrates of circuit boards is selected from ceramics such as alumina, forsterite, steatite, cordierite, mullite and the like in view of their high heat resistance and stability. One of the major problems common in these ceramic materials is that the manufacturing process thereof involves a step of sintering which must be performed at an extremely high temperature of 1200.degree. C. or higher. In this regard, an improvement has been proposed that the sintering temperature can be decreased when an alkali metal constituent and plumbeous constituent are contained in the ceramic material to be sintered. The improvement obtained by such a means is not applicable to ceramic materials for electric use in general because of the adverse influences caused thereby on the mechanical strengths and electrical insulation.
Alternatively, ceramic materials for circuit board substrates sinterable at a decreased temperature have been proposed in Japanese Patent No. Kokai 61-278195, which is a composite of inorganic oxide constituents including alumina Al.sub.2 O.sub.3 and zirconium silicate ZrSiO.sub.4 and a glassy constituent composed of inorganic oxies such as silica SiO.sub.2, alumina Al.sub.2 O.sub.3, boron oxide B.sub.2 O.sub.3, zinc oxide ZnO, barium oxide BaO, magnesium oxide MgO and calcium oxide CaO. An unavoidable defect in the ceramic materials of this type is the relatively low mechanical strengths as a consequence of the difference in the thermal expansion coefficients between the non-glassy sintered phase of the inorganic oxides and the glassy phase in addition to the generally low mechanical strengths of the glassy material.
Several prior art documents disclose similar composite sintered ceramic bodies composed of a glassy phase and a non-glassy or crystalline phase including U.S. Pat. No. 4,650,923 and U.S. Pat. No. 4,490,429, Japanese Patent Nos. Kokai 60-260465, 60-257195 and 62-113758 and Japanese Patent Publications Nos. 59-22399, 60-8229 and 58-28757. The ceramic materials disclosed in these documents, however, have several problems such as a difficulty in controlling the degree of crystallization as a key factor for the mechanical strength and low resistance against migration of metals as well as in the stability at high temperatures due to evaporation loss of some constituents since they contain calcium oxide and/or magnesium oxide as the principal alkaline earth constituents, lead oxide and alkali metal oxides.